Dip-formed synthetic polyisoprene latex articles with improved intraparticle and interparticle crosslinks

ABSTRACT

A synthetic polyisoprene latex emulsion has pre-vulcanization composition and post vulcanization composition. The pre-vulcanization composition comprises soluble sulfur with high S 8  ring structure that is catalytically broken by a zinc dithiocarbamate. Surfactants present in the pre-vulcanization composition wets synthetic polyisoprene particles and permeates small sized sulfur and accelerator molecules into the interior of these particles thereby pre-vulcanizing the particles. The degree of pre-vulcanization is verified by isopropanol index test. The latex emulsion has post-vulcanization composition with accelerators that crosslink inter-particle region during post vulcanization cure cycle. The dipped synthetic polyisoprene article is substantially uniformly cured both in the inter-particle and intra-particle regions and reliably exhibits high cross link density, uniform distribution of double bonds in TEM and zinc segregation at the boundaries or original particles by electron microprobe analysis. The films exhibit high tensile strength, tensile modulus, tear strength, burst pressure and burst volume.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13,277,445, filed Oct. 20, 2011, now U.S. Pat. No. 8,464,719, which is acontinuation of Ser. No. 12,194,118, filed Aug. 19, 2008, now U.S. Pat.No. 8,087,412 which in turn claims priority to U.S. Patent ApplicationSer. No. 61,049,637, filed May 1, 2008. The aforementioned relatedpatent applications are herein incorporated by reference in itsentirety.

FIELD OF THE INVENTION

The invention relates to producing synthetic polyisoprene articles andmethod therefor with improved inter particle and intra particle bondusing controlled pre-vulcanized particles of synthetic latex that is dipformed into a thin latex article from an aqueous latex emulsion.

BACKGROUND OF THE INVENTION

Condoms and gloves are typically made from vulcanized natural rubber.Natural rubber is produced in latex form by the Hevea brasiliensis treeand has unique characteristics. These characteristics make naturalrubber particularly useful for the preparation of barrier protectionproducts. Among the unique characteristics of natural rubber is its highlevel of stereo-regularity, meaning that the polymer of which it iscomprised is a chain consisting almost exclusively of cis-1,4 isopreneunits. Natural rubber latex is also a highly branched polymer with ahigh molecular weight and a wide molecular weight distribution. Thesecharacteristics of the base latex result in vulcanized rubber filmproducts having a unique combination of strength and elasticity.However, natural polyisoprene also contains proteins that have beenshown to produce dermal allergic reaction in some susceptibleindividuals.

Synthetic polyisoprene has been developed to provide a material with thebenefits of natural rubber and to eliminate the potential for proteinallergy. However, development of a true replacement for natural rubberhas proved to be difficult, with synthetic variants such as thatproduced by Kraton Inc. by anionic addition polymerization typically hasa lower level of stereo-regularity (i.e., less than 90% cis 1,4isoprene) and reduced molecular weight characteristics. This molecularcharacter, in turn, has resulted in synthetic polyisoprene films havingan inferior balance of properties compared to those of vulcanizednatural rubber films. Consequently, the addition of a cross-linkingagent tends to produce more inter-particle cross-links and lessintra-particle cross-links during post-vulcanization, resulting innonhomogeneous cure properties leading to latex film articles havingpoor strength and elongation properties, such as voids and cracks due tothe formation of fractures in the inter-particle regions. In addition,synthetic polyisoprene latex flocks more easily, which result in defectsin dipped films, and the latex dip tank has a very limited lifetime thatis available for dipping articles. It is, therefore, imperative thatsynthetic polyisoprene films are cross-linked better to mimic thebranched polymeric structure of a natural rubber, thereby providingimproved properties.

In dip molding processes, the majority of work with synthetic or naturalpolyisoprene has been focused on the development of polyisoprene gloves,using a coagulation dip process. In this type of process, a glove-shapedmold is first dipped into a coagulant solution that is known todestabilize the latex formulation. The resulting coagulant layer is thendried, before the mold is dipped into a bath of a compounded latexformulation to form a coagulated wet latex gel. This coagulated wetlatex gel is typically leached in water to remove residual surfactantbefore being dried at a relatively high temperature to complete thecross-linking of the rubber film. The use of a coagulant layer isundesirable in the manufacture of condoms because it impedes the abilityto produce a thin latex layer and therefore condoms are dipped over acoagulant free former.

The use of vulcanizing or sulfur cross-linking agents in the manufactureof rubber articles is well-known. The effectiveness of sulfurcrosslinking agents is improved by conventional accelerators includingdithiocarbamates, thiazoles, guanidines, thioureas, amines, disulfides,thiurams, xanthates and sulfonamides. The use of vulcanizing agents inthe manufacture of polyisoprene rubber is disclosed in D'Sidocky et al.,U.S. Pat. No. 5,744,552, and Rauchfuss et al., U.S. Pat. No. 6,114,469.

U.S. Pat. No. 3,971,746 to Hirai et al. discloses synthetic polyisoprenerubber latex produced by emulsifying a solution of polyisoprene rubberin an organic solvent including 4-20 wt % of benzene, toluene or xylenewith water. After dipping, the solvent is removed by evaporation fromthe resulting oil-in-water emulsion.

U.S. Pat. No. 4,695,609 to Stevenson discloses vulcanizable rubbercompositions containing less than 0.4 parts by weight of nitrosatablematerials per 100 parts by weight rubber of xanthogen polysulfide andxanthate compounds. This rubber composition contains a dihydrocarbylxanthogen polysulphide and a xanthate selected from metalhydrocarbylxanthates and dihydrocarbylxanthates. While commercialaqueous latex compositions are discussed in Examples 9A-E, the aqueouslatex compositions do not comprise synthetic polyisoprene. Furthermore,the aqueous latex emulsion 9E contains sulfur, zinc oxide and zincdiethyldithiocarbamate, is stable for only four days, and is capable ofproducing a product having a tensile strength at fracture of only 22.4MPa, and an elongation of 830%.

U.S. Pat. No. 5,254,635 to Stevenson discloses a rubber compositioncontaining dibenzylthiuram sulfide. A dibenzylthiuram sulfide, such astetrabenzylthiuram disulphide, is combined with a dihydrocarbylxanthogen polysulphide and/or a xanthate to provide a composition, whichcross-links natural rubber at 120-180° C. without providing harmfulnitrosatables. This natural latex composition, however, is sulfur-freeand does not cross-link intra particle regions of a syntheticcis-1,4-polyisoprene having low levels of stereo-regularity. Therefore,the use of this cross-linking agent package for synthetic polyisoprenelatex will result in a non-uniform article with inferior properties.

U.S. Pat. No. 6,221,447 to Munn et al. discloses the preparation ofhypo-allergenic rubber products, which shrink from a second shape andsize to their original shape and size on application of heat. Theexamples include a polyisoprene condom, which will shrink to fit theindividual user during use. The curing package used to make this condomconsists of agents such as peroxides and/or sulfur.

U.S. Pat. No. 6,391,326 to Crepeau et al. discloses stable emulsions,methods of preparation, and applications, such as in the formation ofelastomeric films. The stable emulsions for preparing an elastomericfilm comprise (1) a phase A containing an elastomer dissolved in anorganic apolar or slightly polar solvent, in which is dispersed (2) aphase B containing a polymer in solution or dispersed in a polarsolvent, which is immiscible with phase A, and (3) a dispersing agentselected from the group consisting of block and grafted polymers.Droplets of phase B having a diameter of 10μ form in phase A. Crepeau etal. does not teach or suggest methods of stabilizing a syntheticpolyisoprene latex emulsion against ‘flock’ formation.

U.S. Pat. No. 6,618,861 to Saks, et al. discloses medical gloves withwatch viewing capabilities. This patent discloses a polyisoprene latexcompound that includes an accelerator system of 2.0 parts per hundred(“phr”) tetramethylthiuram disulfide (“TMTD”), 0.2 phr zinc2-mercaptobenzothiazole (“ZMBT”), 0.2 phr zinc dibutyldithiocarbamate(“ZDBC”), 0.2 phr 1,3-diphenyl-2-thiourea and 0.2 phr zincdiethyldithiocarbamate (“ZDEC”). However, after curing, this acceleratorsystem provides a product having a tensile strength only of about 1,900psi.

U.S. Pat. Nos. 6,653,380 and 7,048,977 to Dzikowicz disclose latex filmcompound with improved tear resistance. The method of enhances the tearresistance, tensile strength, and the aging properties of a latexproduct by adding an antioxidant synergist with an antioxidant to alatex compound. The latex compound comprises a polymer, a stabilizingsystem, a film surface conditioner and a curing system that comprises anactivator, crosslinker and accelerator. Antioxidant synergists include2-mercaptobenzimidazole (MBI), 2-mercaptotoluimidazole (MTI), zinc2-mercaptobenzimidazole (ZMBI) and zinc 2-mercaptotoluimidazole (ZMTI).The latex products formed may be gloves but can also include threads,balloons and other latex-related products. The latex used is notsynthetic polyisoprene and the addition of anti-oxidants does notpre-vulcanize the synthetic polyisoprene latex.

U.S. Pat. No. 6,828,387 to Wang et al. discloses polyisoprene articlesand a process for making the same. This process produces syntheticpolyisoprene articles exhibiting tensile strength properties similar tothose of solvent-based processes using natural rubber latex. The processcombines a synthetic latex with sulfur, zinc oxide and an acceleratorcomposition comprising a dithiocarbamate, a thiazole, and a guanidinecompound, all three of which need to be present, at the pre-cure stage.In a preferred embodiment, the accelerator composition comprises zincdiethyldithiocarbamate (ZDEC), zinc 2-mercaptobenzothiazole (ZMBT), anddiphenyl guanidine (DPG), in conjunction with a stabilizer, which isprimarily milk protein salt, such as sodium caseinate. Polyisoprenelatex (typically 60% solids) and the stabilizer (e.g., sodium caseinate)are combined at ambient temperature (about 20-25° C.). After mixing fora period of time, the mixture is then diluted to 40% solids in water.Wingstay L is then added, and the mixture is stirred for approximately15 min. At this point, the pH can be adjusted to a range of about 8.5 to9.0. Zinc oxide is added, followed by the sulfur and acceleratorcompounds. The elastomeric polyisoprene product made by the process is asurgeon's glove dipped over a coagulant-coated former. The aqueous latexemulsion is stable with a maximum stability of eight days. The tensilestrength of the surgical glove product obtained is approximately 3,000psi (20.6 MPa) (according to ASTM D412). The accelerators are added tothe latex emulsion, but maintained at a low temperature for up to eightdays. The dithiocarbamate, a thiazole and a guanidine accelerators mustbe present in the latex together. The latex stabilizer is sodiumcasinate. The stability of this aqueous latex composition is better thanthat of Stevenson (U.S. Pat. No. 4,695,609). The glove formers aredipped in a coagulant solution containing calcium nitrate that isunsuited for coagulant-free dipping of synthetic polyisoprene latexcondom.

U.S. Pat. No. 7,041,746 to Dzikowicz discloses accelerator system forsynthetic polyisoprene latex. The accelerator system comprisesdithiocabamate and thiourea and can produce synthetic polyisoprene filmshaving a tensile strength of about 3,000 psi to about 5,000 psi at lowcuring temperatures. The accelerator system does not containtetramethyithiuram disulfide or diphenylguanidine or sodiumdibutyldithiocarbamate (SDBC), or diisopropyl xanthogen polysulphide(DXP) but contains thiourea. The accelerators are not indicated topre-vulcanize the synthetic polyisoprene particles and the latex articleproduced has a very low modulus of 1.5 MPa at 300% elongation and atensile strength of 20.6 to 34.4 MPa.

UK patent application GB 2,436,566 to Attrill et al. disclosesminimizing pre-vulcanization of polyisoprene latex. This process formaking a polyisoprene latex comprises compounding a syntheticpolyisoprene latex with compounding ingredients and maturing the latexat a low temperature so as to minimize pre-vulcanization. Dipping ofcondoms is also conducted at low temperatures typically 15° C. to lessthan 20° C. The absence of pre-vulcanization is verified be assuring thestrength of a ring made has a prevulcanisate relaxed modulus has a valueless than 0.1 MPa indicative of the absence of pre-vulcanization. Thelatex emulsion may contain accelerator such as dithiocarbamate. The '566patent application teaches away from pre-vulcanization prior to dippingof latex articles.

There is a need, therefore, for a stable synthetic polyisoprene latexemulsion composition that does not agglomerate or flock, providingusable emulsion lifetimes. The composition should achieve substantialintra-particle and inter-particle crosslinking in the final product.Such a composition would enable the dip-forming of articles in theabsence of a coagulant, such that articles having thinner, continuous,and defect-free layers with enhanced strength and improvedstretchability could be obtained. Such articles would not deteriorateand would maintain their physical integrity over time. It is an objectof the present invention to provide such a composition, as well as amethod of preparing and using such a composition to dip-form articles,and the articles so produced. These and other objects and advantages, aswell as additional inventive features, will become apparent from thedetailed description provided herein.

SUMMARY OF THE INVENTION

The present invention provides a latex article that is formed by dippinga condom shaped former in a pre-vulcanized synthetic latex emulsionwithout use of any coagulants and curing the condom thus produced.Synthetic polyisoprene latex is available from Kraton, which is producedby anionic polymerization with a high cis-1,4 content. The syntheticlatex particles in the latex emulsion are pre-vulcanized by theincorporation of sulfur within the interstices of latex particles. Thisincorporation of sulfur within the synthetic latex particles isaccomplished by 1) using a sulfur emulsion that has a high content ofsoluble sulfur with S₈ ring structure; 2) said ring structure beingdisrupted or broken by catalytic activity of zinc dithiocarbamateresulting in linear sulfur chains in the latex emulsion adapted for easymigration into the particles of synthetic polyisoprene in the latexemulsion; 3) using a potassium caprylate surfactant and sodium dodecylbenzene sulphonate (SDBS) surfactant to wet the particles of syntheticpolyisoprene in the latex emulsion there by chains of sulfur along withsulfur captured zinc dithiocarbamate is available for permeation intosaid particles; 4) allowing sufficient time at selected processtemperature in the range of 20° C. to 30° C. to progressively permeatesulfur into said synthetic polyisoprene particles; 5) validating sulfurpermeation and pre-vulcanization by isopropanol index test wherein thesynthetic polyisoprene particles are no longer very tacky but exhibitslesser degree of tackiness with an isopropanol index of 3. Zincdithiocabamate is a zinc complex of dithiocarbamate and includes zincdimethyldithiocarbamate, zinch diethyl dithiocarbamate, zincdibutyldithiocarbamate. In addition, the synthetic polyisoprene latexemulsion has other crosslinking agents such as sodiumdibutyldithiocarbamate (SDBC), tetrabenzyl thiuram disulfide,diisopropyl xanthogen, tetraethylthiuram disulfide, xanthogen sulfidefor curing the inter-particle regions during the vulcanization or curecycle. Insoluble sulfur such as amorphous sulfur or polysulfur presentin the sulfur added to the latex emulsion becomes soluble atpost-vulcanization cure temperature and reacts with zinc dithiocarbamateaccelerator curing inter-particle regions. During post vulcanizationcure, pre-vulcanized synthetic polyisoprene particles with the permeatedsulfur also cure completely in the intra-particle regions. Thereforeusing this methodology of using a pre-vulcanization accelerator packageand post vulcanization accelerator package a substantially uniform curedsynthetic latex condom film is produced.

The product thus produced has several distinguishing features that haveimprints of this pre-vulcanization and post-vulcanization methodology.Since the synthetic polyisoprene thin film of latex is cured withimproved crosslink density, the molecular weight between crosslinksexhibits a lower value. Since zinc complex of dithiocarbamatecatalytically breaks the S₈ ring of sulfur and as a catalyst, it isavailable for subsequent use and does not readily penetrate thesynthetic polyisoprene due to its large molecular size. The molecularsize of zinc dibutyldithiocarbamate is a larger than that of zincdimethyldithiocarbamate which has a molecular size greater than that ofzinc dimethyldithiocarbamate. Zinc dibenzyldithiocarbamate and zincdiphenyldithiocarbamate are even larger molecules and will resistpermeation into the synthetic polyisoprene latex particles. Thus thepreferred zinc complex of dithiocarbamate for pre-vulcanization ofsynthetic latex particles in the latex emulsion is zincdibutyldithiocarbamate (ZDBC) or zinc diethydithiocarbamate (ZDEC).There is an accumulation of zinc containing compound surrounds each ofthe original synthetic polyisoprene particles, and this microstructuralfeature can be readily observed by microprobe elemental analysis usingan electron microscope. The synthetic polyisoprene films producedtypically have high tensile strength, high tensile modulus andelongation at fracture with the fracture front passing through both theinter particle and intra particle regions indicating that the intraparticle regions and inter particle regions are substantially of equalstrength within the synthetic latex films produced.

The method for producing synthetic polyisoprene products comprises useof a synthetic latex emulsion that includes a pre-vulcanizationcomposition and post-vulcanization composition along with conventionallatex emulsion additives comprising stabilizers, pH control agents,antioxidants, preservatives etc. Preferably, the synthetic polyisopreneparticles are cis-1,4-polyisoprene, have a diameter in the range ofabout 0.2 to 2 micrometers, and are maintained in an aqueous medium ofthe latex emulsion. Kraton® ‘IR-KP401A’ latex is supplied by KratonPolymers Group, 15710 John F. Kennedy Blvd., Suite 300, Houston, Tex.77032 and has these properties. The pre-vulcanization composition hassulfur with high soluble sulfur content, typically of the S₈ ringstructure, zinc dithiocarbamate accelerator that can break or disruptthe S₈ sulfur ring structure, a combination of surfactants includingpotassium caprylate also known as potassium salt of octanic acid andsodium dodecyl benzene sulphonate (SDBS). Reference to “high solublesulfur content” means having enough soluble sulfur present to formsufficient to permeate into latex particles in the aqueous latexemulsion and crosslink during curing to achieve commercially acceptablearticles such as condoms and/or gloves. The pre-vulcanization of thesynthetic latex particles in the latex emulsion occurs over a period oftime between 9 hours to 2 days depending on the temperature of the latexemulsion which is generally in the range of 20° C. to 30° C. The degreeof pre-vulcanization of the synthetic latex particles is monitored by anisopropanol index test and the latex particles progress from a verytacky feel (index ˜1.0) to a lesser degree of tacky feel (index 3) aspre-vulcanizing sulfur is incorporated within the particle. Thepost-vulcanization composition includes amorphous or polysulfur, whichis insoluble at latex emulsion temperature but is soluble atvulcanization or cure temperature. Other accelerators in the syntheticaqueous latex emulsion includes, but are not limited to zincdiethyldithiocarbamate (ZDEC), to zinc dibutyldithiocarbamate (ZDBC),sodium diethyldithiocarbamate (SDEC), sodium dibutyldithiocarbamate(SDBC), a thiuram compound and diisopropyl xanthogen polysulphide (DXP).Zinc oxide may also be added as an activator.

A typical synthetic polyisoprene latex emulsion composition is providedin terms of 100 parts by weight of dry rubber (phr). The pre-vulcanizingcomposition includes sulfur in the range of 0.6 to 1.8 wt %; Acceleratorpackage includes ZDEC and/or ZDBC accelerator SDBC accelerator, DXPaccelerator together with reactive zinc oxide activator is used with atotal accelerator content in the range of 0.6 wt % to 2.5%. Thesurfactant package includes potassium caprylate, sodium dodecyl benzenesulphonate and polyoxyethylene cetyl/stearyl ether with surfactants inthe range of 0.3 to 1.5 wt %; Wingstay L or butylated reaction productof p-cresol & dicyclopentadiene anti-oxidant preservative is in therange of 0.3 to 1 wt %; ammonium hydroxide is in the range of 0 to 0.36wt %. As indicated earlier, the pre-vulcanization composition of thesynthetic polyisoprene latex composition includes soluble sulfur, ZDECand/or ZDBC accelerator, potassium caprylate surfactant and SDBSsurfactant and polyoxyethylene cetyl/stearyl ether surfactant. Thepost-vulcanization composition includes sulfur especially that which isinsoluble, SDBC accelerator, DXP accelerator, ZDEC and/or ZDBC. Thepre-vulcanization composition provides the availability of sulfur tosynthetic polyisoprene latex particles in the aqueous syntheticpolyisoprene emulsion pre-vulcanizing the intra-particle regions and theentire particle of synthetic polyisoprene is crosslinked duringvulcanization cure cycle. The post-vulcanization composition providesthe ability to crosslink regions between the particles of syntheticpolyisoprene or inter-particle regions thereby assuring a high qualitysubstantially uniformly cured synthetic polyisoprene product.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transmission electron micrograph of the middle portion of apre-vulcanized and post-vulcanized synthetic polyisoprene latex filmprepared in accordance with the present invention, wherein 10 showsuniformly distributed, cross-linked networks of polyisoprene particles,11 shows an inter-particle region, which also evidences uniformlydistributed cross-linking, and 13 shows remnants of polystyrene, whichis used to swell the film in preparation for the micrograph.

FIG. 2 is a scanning electron micrograph of a cross section of asynthetic polyisoprene condom that was frozen in liquid nitrogen andcleaved. The sample was coated with a thin film of iridium to preventcharging of the insulating latex rubber condom by electron beam. Due tothe low temperature of liquid nitrogen, the synthetic polyisoprenecondom material behaved as a brittle solid showing conchoidal orshell-like fracture surfaces along X1-X1 and X2-X2. There were no grainsvisible in this fracture surface, indicating that the fracture strengthat the intra grain region and inter grain region was very nearly thesame and the therefore the fracture surface was nearly isotropiceverywhere. A dimensional marker shows a line, which is calibrated to be20 microns in length.

FIG. 3 is an x-ray map of chemical elements present in the sample shownin FIG. 2. It shows one or more x-ray peaks for Zn, S and Ir in additionto carbon.

FIG. 4A is a set of three photographic images. A first image provides ascanning electron micrograph of the cross-section show in FIG. 2 at aselected location near the upper marking of X1. An image of a zinc x-raymap in the same area and an image of a sulfur x-ray map are also shown.It is recognized that generally, upon creation, x-ray maps of zinc andsulfur are usually a black background photograph with zinc or sulfurx-ray beams emitted from the sample providing a series of white dots.The zinc map and sulfur map images of FIG. 4A, however, were inverted incontrast for clarity. A selected region marked P1 is shown in all threeimages. As seen in the zinc x-ray map, the region P1 encompasses aseries of zinc black dots that define a region, with no zinc black dotswithin the region. The corresponding sulfur x-ray map shows plurality ofsulfur black dots. From this image, it is concluded that this was asingle grain of polyisoprene particle in the polyisorene latex emulsion.It is also concluded that during the pre-vulcanization stage, the sulfurmolecule was catalyzed by the ZDBC allowing sulfur to enter thepolyisoprene particle, as seen in the sulfur x-ray map. The zinc, on theother hand, was left behind due to the large molecular size of ZDBCdecorating the exterior of the polyisoprene particle, as seen in thezinc x-ray map. FIG. 4B shows the x-ray maps of zinc and sulfur in theregion P1 that have been magnified for clarity, where the zinc blackdots and sulfur black dots are clearly visible. The polyisopreneparticle in the region P1 has an approximate dimension of 4 microns.

FIG. 5 is a scanning electron micrograph of the fracture surface of acondom that was ruptured by blowing high pressure nitrogen to form aballoon that eventually burst. This test was done at room temperature.The sample was coated with a thin film of iridium to prevent charging ofthe insulating latex rubber condom by electron beam. The fracturesurface as shown in this figure shows a fracture surface that was verynearly planar with no features indicating intra particle or interparticle regions. This absence of intra-polyisoprene particle andinter-polyisooprene particle features means that the fracture surfacepropagated with no preference for either the intra particle region orthe inter particle regions indicating that both inter and intra particleregions were approximately equal strength or were crosslinked nearlyequally. The latex condom fractured at room temperature as an elasticsolid showing planar fracture surface, not a conchoidal or shell-likefracture surface. There were no grains visible in this fracture surface,indicating that the fracture strength at the intra grain region andinter grain region was very nearly the same and the therefore thefracture surface was nearly isotropic everywhere. A dimensional markershows a line, which is calibrated to be 20 microns in length.

FIG. 6 is an x-ray map of chemical elements present in the sample shownin FIG. 5. It shows one or more x-ray peaks for Zn, S and Ir in additionto carbon.

FIG. 7A is a set of three photographic images. A first image provides ascanning electron micrograph of the fracture at a selected location nearthe circular feature near the central location of FIG. 5. An image of azinc x-ray map in the same area and an image of a sulfur x-ray map arealso shown. It is recognized that generally, upon creation, x-ray mapsof zinc and sulfur are usually a black background photograph with zincor sulfur x-ray beams emitted from the sample providing a series ofwhite dots. The zinc map and sulfur map of FIG. 7A, however, wereinverted in contrast for clarity. A selected region marked P2 is shownin all three images. As seen in the zinc x-ray map, the region P2encompasses a series of zinc black dots that define a region, with nozinc black dots within the region. The corresponding sulfur x-ray mapshows plurality of sulfur black dots. From this image, it is concludedthat this was a single grain of polyisoprene particle in the polyisorenelatex emulsion. It is also concluded that during the pre-vulcanizationstage, the sulfur molecule was catalyzed by the ZDBC allowing sulfur toenter the polyisoprene particle, as seen in the sulfur x-ray map. Thezinc, on the other hand, was left behind due to the large molecular sizeof ZDBC decorating the exterior of the polyisoprene particle as seen inthe zinc x-ray map. FIG. 7B shows the x-ray maps of zinc and sulfur inthe region P2 that have been magnified for clarity, where the zinc blackdots and sulfur black dots are clearly visible. The polyisopreneparticle in the region P2 has an approximate dimension of 4 microns.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is predicated on the discovery that solublesulfur, such as S₈ rings of sulfur, is catalyzed by a zinc complex ofdithiocarbamate in combination with potassium caprylate and sodiumdodecyl benzene sulphonate (SDBS) surfactant creating pre-vulcanized,synthetic polyisoprene particles in a latex composition. This latexcomposition enables the production of latex film articles by dippingcoagulant coated or coagulant free formers into the composition. Asurfactant package inhibits synthetic polyisoprene particleagglomeration and flocculation. The latex dipped film has syntheticpolyisoprene particles that become crosslinked and regions between theparticles are crosslinked during the vulcanization cure forming bothintra-crosslinked and inter-crosslinked bonds. The articles that resultcomprise a high quality and uniform latex film.

The latex-stabilizing composition is one that keeps the particles ofsynthetic polyisoprene separated from each other in the aqueous medium.Since the polyisoprene particles do not touch each other, they areunable to agglomerate and flock. This is important because, once theparticles begin to agglomerate, the particles may never be separated dueto van der Waals forces. Preferably, the latex-stabilizing compositioncomprises a surfactant package comprising at least one surfactant. Ananionic surfactant is preferred, especially one that can be stablymaintained for a period of well over one month and up to two months ormore. An example of such a surfactant is sodium dodecyl benzenesulphonate (SDBS). Other examples include, but are not limited to, otheralkyl aryl sulphonates, alkyl sulphonates, olefin sulphonates (e.g., C14olefin sulphonate, which is sold under the trade name Calsoft AOS-40(Pilot Chem. Co., Red Bank, N.J.), and alcohol sulphates (e.g., sodiumlauryl sulphate). SDBS or another alkyl aryl sulphonate is preferablypresent in an amount of about 0.1-0.35 wt %, based on the dry weight ofthe polyisoprene. SDBS or another alkyl aryl sulphonate can be combinedwith one or more other surfactants, such as potassium caprylate,polyoxyethylene cetyl/stearyl ether, and the like. For example, SDBS oranother alkyl aryl sulphonate can be combined with potassium caprylate,alone or in further combination with polyoxyethylene cetyl/stearylether. When SDBS or another alkyl aryl sulphonate is used in combinationwith one or more other surfactants, preferably each surfactant ispresent in an amount of about 0.05-1.2 wt %, based on the dry weight ofthe polyisoprene, and the total amount of the surfactant package isabout 0.4-1.2 wt %, based on the dry weight of the polyisoprene. WhenSDBS or another alkyl aryl sulphonate is used in combination withpotassium caprylate and polyoxyethylene cetyl-stearyl ether, preferablythe polyoxyethylene cetyl-stearyl ether is present in an amount of about0.1-0.5 wt %, based on the dry weight of the polyisoprene.

In view of the above, the present invention provides asurfactant-stabilized, pre-vulcanized, synthetic polyisoprene latexcomposition having a isopropanol index rating of 3.0. The isopropanolindex test measures the extent of pre-vulcanization of synthetic latexparticles in an aqueous latex emulsion by combining equal volumes oflatex and isopropanol at room temperature and allowing the mixture tostand for 3 min. The isopropanol coagulates the latex, and the resultingconsistency is numerically rated. The consistency of the coagulumindicates the degree of pre-vulcanization of the latex. As the latexbecomes more pre-vulcanized, the coagulum loses more of its tackinessand becomes more crumbly. A rating of 2.5 indicates that small lumpsform, whereas a rating of 3.0 indicates that the lumps are non-tacky, arating of 3.5 indicates that, not only are the lumps non-tacky, thelumps disintegrate easily, and a rating of 4.0 indicates that dry crumbsform, evidencing a high degree of pre-vulcanization of the syntheticlatex particles. The pre-vulcanization is monitored to assure that thesynthetic latex emulsion is ready for dipping of polyisoprene condoms.

The pre-vulcanization composition includes potassium caprylate and SDBSor another alkyl aryl sulphonate surfactants with zinc dithiocarbamateand soluble sulfur. The latex emulsion with surfactants wets thesynthetic polyisoprene particles, catalytic action of zincdithiocarbamate breaks the ring of soluble S₈ molecule forming linearchain of soluble sulfur pre-vulcanizing particles of syntheticpolyisoprene. The post-vulcanization composition has sulfur and otheraccelerators that cause inter-particle cross-linking duringvulcanization cure. Such cross-linking results in a more homogeneouslatex film having greater strength and elongation properties andcrosslink density.

Preferably, the pre-vulcanizing composition comprises (i) across-linking package comprising zinc diethyldithiacarbamate or zincdibutyldithiocarbamate accelerator and soluble sulfur (ii) a wettingagent. During pre-vulcanization, sulfur with its ring structure brokenby the catalytic action of zinc dithiocarbamate accelerator penetratesthe polyisoprene particles and initially interacts with the isoprenedouble bonds therein. The catalytic reactivity of zinc dithiocarbamateis detailed in the publication entitled “The Mechanism ofZinc(II)-Dithiocarbamate-Accelerated Vulcanization Uncovered;Theoretical and Experimental Evidence” by Nieuwenhuizen, et al. ispublished in J. Am. Chem. Soc., 121 (1), 163-168, 1999. A secondpublication entitled “Zinc accelerator complexes. Versatile homogeneouscatalysts in sulfur vulcanization” by Nieuwenhuizen published in AppliedCatalysis A: General 207 (2001) 55-68. These two publications discussthe mechanism of catalytic action of zinc dithiocarbamates specificallyzinc dimethyldithiocarbamate with sulfur. The book published by Gary R.Hamed, professor at University of Akron, the chapter 2 of which isavailable at web addressfiles.hanser.de/hanser/docs/20040401_(—)244515439-6683_(—)3-446-21403-8.pdfclearly indicates in Chapter 2.3.1.1. that for sulfur to be soluble itmust have S₈ rings. The same chapter indicates that with ZDBC, you needonly small amount of sulfur since ZDBC is an ultrafast accelerator. Theweb article at http:/www.chemistrymag.org/cji/2007/097032pe.htm entitled‘Effect of adding pyridine ligand on the structure and properties ofcomplex Zn(S₂CNBz₂)₂’ by Zhong et al. indicates that zincdibenzyldithiocarbamate and zinc dipyridinedithiocarbamate also havesimilar functionality of catalytic activity with sulfur.

It is recognized that, unlike the S₈ rings of soluble sulfur, amorphousor polymeric sulfur are not soluble. However amorphous or polymericsulfur becomes soluble at 120° C., which is at or near the latex curetemperature, thus insoluble or polymeric sulfur remain outside syntheticpolyisoprene particles in the latex emulsion and facilitatescrosslinking of inter particle regions. According to embodiments of thepresent invention, diffusion of sulfur into synthetic polyisopreneparticle requires sulfur to be soluble. The wetting agents used inaccordance with the present invention facilitate wetting of thepolyisoprene particles and brings soluble sulfur with ring structurebroken by zinc dithiocarbamate catalytic action into contact with thesurface of the polyisoprene particles and permeation of sulfur occursduring processing time provided. The pre-vulcanized structure of theaqueous latex emulsion is stable for several days, e.g., up to 5 days.

Sulfur is preferably present in the synthetic polyisoprene latexemulsion in an amount of about 0.8-1.8 wt %, based on the dry weight ofpolyisoprene. If zinc oxide is used, preferably it is present in anamount of about 0-0.5 wt %, based on the dry weight of polyisoprene.

Examples of suitable wetting agents include, but are not limited to,salts (e.g., sodium salt or potassium salt) of fatty acids, which areanionic, e.g., sodium stearate, sodium oleate, and potassium caprylate.Potassium caprylate is advantageously used with a salt of a short-chainfatty acid, SDBS and polyoxyethylene cetyl/stearyl ether.

The penetration of the components of the pre-vulcanizing compositioninto the polyisoprene particles is a strong function of the polyisopreneparticle size and size distribution. Typically, smaller particles have alarger surface area, and the components of the pre-vulcanizingcomposition penetrate these small particles more rapidly. However, theselarger surface areas result in more inter-particle regions, which arecross-linked by the cross-linking agent during post-vulcanization. Incontrast, larger particles have a smaller surface area, and thecomponents of the pre-vulcanizing composition penetrate these largeparticles more slowly. The smaller surface areas result in lessinter-particle regions. Aggregates of smaller particles appear like alarge particle, which behaves differently than a large particle.Therefore, there is a delicate balance in selecting the size and sizerange distribution of the polyisoprene particles to produce optimalstrength properties that balance pre-vulcanization intra-particlecross-linking with post-vulcanization inter-particle cross-linking. Asindicated above, particles in the range of about 0.2-2 micrometersprovide optimal results. The penetration of the components of thepre-vulcanizing composition into the polyisoprene particles is also afunction of the diffusion process, itself, which is a linear function oftime and an exponential function of temperature, reflecting a thermallyactivated process. Therefore, increasing the temperature by a fewdegrees during the pre-vulcanization step increases significantly thepre-vulcanization rate. For example, pre-vulcanization at roomtemperature requires from about 3-5 days or as much as about 9 days,while pre-vulcanization at an elevated temperature, e.g., about 50-70°C., requires only about 3-7 hours.

Preferably, the post-vulcanization composition comprises sodium dibutyldithiocarbamate (SDBC), sulfur, a thiuram compound, and/or a xanthogencompound, alone or in further combination with a surfactant. Examples ofsuitable xanthogens include, but are not limited to, diisopropylxanthogen polysulphide (DXP), diisopropyl xanthogen, tetraethylthiuramdisulfide, and xanthogen sulfide. DXP is a preferred xanthogen. Anexample of a thiuram compound is tetrabenzyl thiuram disulfide. Thepost-vulcanization composition is one that causes inter-particlecross-linking upon activation at the elevated temperature (e.g.,120-150° C.). In addition, this post-vulcanization cure also crosslinksthe synthetic polyisoprene particles with permeated sulfur. Suchcrosslinking results in a more homogeneous latex film having greaterstrength and elongation properties.

The method comprises adding a latex-stabilizing composition, such as onecomprising a surfactant package comprising at least one surfactant, suchas at least one surfactant selected from the group consisting of analkyl aryl sulphonate (e.g., SDBS), an alkyl sulphonate (e.g., olefinsulphonate) and an alcohol sulphate (e.g., sodium lauryl sulphate). SDBScan be combined with potassium caprylate, alone or with polyoxyethylenecetyl/stearyl ether. A preferred surfactant package comprises SDBS,potassium caprylate, and polyoxyethylene cetyl/stearyl ether. Uponaddition of the latex-stabilizing composition, the emulsion is stirred,to keep the polyisoprene particles from touching each other.

Then, the method comprises the steps of adding a pre-vulcanizationcomposition to formulate a synthetic polyisoprene latex emulsion (a) azinc dithiocarbamate selected from zinc diethyldithiocarbamate and zincdibutyldithiocarbamate and combinations thereof; (b) sulfur, preferablywith high S₈ content and (b) a wetting agent. The wetting agent ispreferably a salt of a fatty acid, such as sodium stearate, sodiumoleate, or potassium caprylate. The aqueous latex emulsion is stirredand periodically examined for permeation of pre-vulcanization agentsinto the synthetic polyisoprene particles by using the isopropanol indextest. The reason why this sequence is adopted is because thepolyisoprene latex has an inherent tendency to flock and ‘case harden’due to peripheral reaction with sulfur catalyzed by ZDBC or ZDEC. Thishas to be prevented so that tightly bonded particles do not result. Thepresence of surfactants and creation of opened out S₈ chains of sulfurenables the diffusion of sulfur into the particles.

The method further comprises the steps of adding post-vulcanizationcomposition to the synthetic polyisoprene latex emulsion withaccelerators selected from the group consisting of SDBC, reactive zincoxide, sodium diethyldithiocarbamate, sodium dibutyldithiocarbamate,thiuram such as tetrabenzyl thiuram disulfide and xanthogen. If reactivezinc oxide is present, preferably it is present in an amount of about 0to 0.5 wt %, based on the dry weight of polyisoprene. The thiuram can betetraethylthiuram disulfide, tetrabenzyl thiuram disulfide. Thexanthogen can be DXP, diisopropyl xanthogen, or xanthogen sulfide. Thecomposition thus produced is stable for up to about 5 days at 20° C. to25° C. and can be used in a production line.

Table 1 below shows an example of a composition that exhibitspre-vulcanization behavior.

TABLE 1 Quantity per hundred dry Formulation rubber (phr) IR-KP 401Kraton Latex 100 alkyl aryl sulphonate 0.1-0.3 potassiumcaprylate/potassium  0.1-0.46 oleate polyoxyethylene cetyl/stearyl0.1-0.5 ether sulfur 0.8-1.8 reactive zinc oxide 0.05-0.5  ZDEC/ZDBC0.4-1.0 SDBC/SDEC 0.05-0.5  DXP/diisopropyl xanthogen/ 0.2-0.6 xanthogensulfide Wingstay L 0.5-1.0

A typical mixing sequence of the aqueous synthetic latex emulsion isillustrated in Table 2. The table lists the steps and the time periodinvolved.

TABLE 2 Phase I: Addition of chemicals for pre- 5 to 9 days vulcantionincluding sulfur, ZDEC/ ZDBC, surfactant package with potassiumcaprylate and polyoxyethylene cetyl/stearyl ether Phase II: addition ofpost vulcanization Prior to dip accelerators including DXP, SDBC, SDEC,tetrabenzyl thiuram disulfide and surfactants

Thus, the present invention further provides a method of forming asynthetic polyisoprene latex article. The method comprises dipping acoagulant-free or coagulant coated former in the above-describedpre-vulcanized synthetic polyisoprene aqueous latex emulsion compositionat least once to form a thin layer of latex film with individualparticles of pre-vulcanized synthetic polyisoprene on the surface of theformer. The former can be any suitable former as is known in the art.The present inventive composition is particularly useful for layeringonto formers for condoms and gloves.

The method then comprises allowing the thin layer of latex film formedon the surface of the former to dry after each dip. The spaces betweenthe particles decrease as the layer dries. After the last layer of latexfilm is dry in the case of multiple dips of the former into thesynthetic polyisoprene latex emulsion, the method further comprisespost-vulcanizing the thin latex film on the former. The film can bepost-vulcanized by heating the film, e.g., to about 120 to 150° C. forabout 8 to 15 min. During this period, the inter-particle regions arecross-linked. The intra-particle regions also undergo furthercrosslinking, producing a more homogeneous latex product. Then, themethod comprises stripping the latex film from the former.

In the absence of pre-vulcanization of the synthetic polyisopreneparticles, crosslinking predominantly occurs in the periphery of thesynthetic polyisoprene particles, resulting in weak particles. Attemptsto crosslink the inter particle region within the particles only duringpost-vulcanization results in over crosslinking of the intra-particleregions, which, in turn, results in a latex product with poor stretchproperties.

Table 3 lists a typical dipping sequence of a condom. A similar sequencecan be created for a synthetic polyisoprene surgical glove.

TABLE 3 First dip (thickness of the film is controlled by total solidscontent of the latex in the dip tank, the latex viscosity and the speedof the formers). Drying of the latex film (60-80° C.; 1-3 min). Seconddip (thickness of the film is controlled by total solids content of thelatex in the dip tank and the speed of the formers). Drying of the latexfilm (60-80° C.; 1-3 min). Beading/ring formation on the open end of thecondom Drying of the ring and latex film (70-100° C., 1-3 min) Curing(110-130° C.; 11-15 min) Leaching (70-80° C., 1-2 min) Stripping of thecondoms from the glass formers

The sequence of dipping for the condoms using the surfactant-stabilized,pre-vulcanized synthetic polyisoprene latex composition is typicallywithin the 5-day period, the average lifetime of synthetic polyisoprenelatex emulsion tank. A condom former is dipped in the composition in afirst dip, and the thickness of the latex film is controlled by thetotal solids content of the composition in the dip tank and the speed ofmovement of the formers. The latex film is dried at about 60-80° C. forabout 1-3 min. The latex film on the former is dipped again into thecomposition to apply a second dip coating. The latex film after thesecond dip is dried at about 60-80° C. for about 1-3 min. The free endof the condom is rolled to create a bead ring and is dried at about70-100° C. for about 1-3 min. The latex film is post-vulcanized at about110-130° C. for about 11-15 min. The latex film is leached in water atabout 70-80° C. for about 1-2 min to remove residual surfactants andcross-linking agents from the latex film. The latex film is thenstripped from the formers. The latex articles produced display higherstrength and improved stretch, even when a low stereo-regularitysynthetic polyisoprene is used. The synthetic polyisoprene articles arefree from irritation-causing proteins and solves the long outstandingproblem of latex sensitivity.

Mechanical properties of a synthetic polyisoprene latex film producedaccording to the subject invention were compared that disclosed in priorart. For example, the synthetic polyisoprene disclosed in U.S. Pat. No.6,828,387 (Wang) had a tensile strength of over 3000 psi (20.68 MPa),elongation of greater than about 750% at break, and a tensile modulus ofless than about 300 psi (2.07 MPa) at 300% elongation as measured inaccordance with ASTM D412.

Tensile Properties of synthetic polyisoprene production condom measuredaccording to ISO 4074:2002 test method is shown in the Table 4 below.

TABLE 4 Batch No. Aged/ Tensile Elongation at Modulus at Condom TypeUnaged Strength MPa Break % 500% MPa 0606030106/Syn. unaged 35.78 10301.57 Polyisoprene 0606040116/Syn. unaged 34.64 1022 1.38 Polyisoprene0606050116/Syn. unaged 30.66 1017 1.38 Polyisoprene 0606030106/Syn. 7days 35.91 1033 1.39 Polyisoprene 70° C. 0606040116/ 7 days 34.43 10211.39 Syn. Polyisoprene 70° C. 0606050116/Syn. 7 days 35.72 1050 1.44Polyisoprene 70° C. Natural Rubber unaged 29 800 2 Natural Rubber 7 days30 800 2.1 70° C.

Tear is a very important property of a condom material. Tear strength ofsynthetic polyisoprene condom was measured and compared with that ofnatural rubber condom according to ASTM D624: 2000 method and is shownin Table 5 below.

TABLE 5 Median Average Tear Median Tear Sample Strength Tear StrengthAverage Batch No. Description N/mm Force N N/mm Tear Force N 612141816Natural 54.26 3.62 53.29 3.57 Rubber Unaged 612141816 Natural 46.67 3.3646.15 3.29 Rubber Aged 7 days 70° C. 606040116 Synthetic 34.83 2.54 34.62.52 polyisoprene Unaged 606040116 Synthetic 34.13 2.33 34.65 2.37polyisoprene Aged 7 days 70° C.

The burst pressure and burst volume of a condom is a critical measure ofits performance. Tables 6A and 6B show burst volume and burst pressuredata.

TABLE 6A Burst Unaged (200 pieces tested) Con- Ei- dom Condom MV NCV MPNCP ther Type Batch (L) SD-V (pcs) (kPa) SD-P (pcs) (pcs) Syn-080201PI16 53.20 3.79 0 1.70 0.13 0 0 thetic PI Syn- 080202PI16 54.094.29 0 1.72 0.15 2 2 thetic PI Syn- 080203PI16 50.77 3.75 0 1.81 0.15 11 thetic PI Natural 0704150316 36.67 2.36 0 2.18 0.11 0 0 Rubber Natural0704590316 34.40 2.39 0 2.13 0.14 0 0 Rubber

TABLE 6B Burst Aged 7 days 70° C. (200 pieces tested) Con- Ei- domCondom MV NCV MP NCP ther Type Batch (L) SD-V (pcs) (kPa) SD-P (pcs)(pcs) Syn- 080201PI16 46.73 3.41 0 1.57 0.12 1 1 thetic PI Syn-080202PI16 49.25 3.42 0 1.54 0.12 0 0 thetic PI Syn- 080203PI16 47.244.35 0 1.51 0.14 4 4 thetic PI Natural 0704150316 36.43 2.05 0 2.05 0.120 0 Rubber Natural 0704590316 29.30 2.88 2 2.06 0.21 0 2 RubberWhere MV, P=Mean Volume, Pressure respectively, SD-V, P=StandardDeviation Volume, Pressure respectively, NCV, P=Nonconformance Volume,Pressure respectively

The method of measuring molecular weight distribution and calculatingcrosslink density requires cutting of disks from condom samples andswelling the disk samples in toluene until equilibrium. The disks wereinitially weighed and after swelling they are weighed again. Theequilibrium volume fraction of the swelled rubber was calculated usingequation shown below. In this equation P_(r) is the density of rubber(0.92 g/cm³), P_(s) is the density of toluene (0.862 g/cm³), W_(r) isthe weight of rubber before swelling and W_(s) is the weight of swelledrubber.

$\frac{\frac{W_{r}}{P_{r}}}{\frac{W_{r}}{P_{r}} + \frac{W_{s} - W_{r}}{P_{s}}}$

The volume fraction was used in the Florey-Rehner equation shown belowto calculate the crosslink density. In this equation n is the crosslinkdensity, V_(s) is the molar volume of toluene the swelling solvent,which is 106.3 cm3/mol, V_(r) is the volume fraction of the rubber phasein the swollen gel, and is the toluene-cis polyisoprene interactionparameter, which is 0.39.

$n = {\frac{1}{V_{s}}\frac{\left\lbrack {{\ln\left( {1 - V_{r}} \right)} + V_{r} + {\chi\; V_{r}^{2}}} \right\rbrack}{\left\lbrack {V_{r}^{\frac{1}{3}} - {0.5\; V_{r}}} \right\rbrack}}$

The molecular weight between crosslinks was calculated by the followingequation.

$M_{c} = \frac{P_{r}}{n}$

Table 7 shown below reports measured molecular weight between crosslinksand corresponding crosslink density for several of syntheticpolyisoprene condoms manufactured according the embodiments of thesubject invention. Also shown are the values for a syntheticpolyisoprene condom marketed by Durex, presumably manufactured accordingto UK GB 2,436,566 LRC patent application. Also shown are the values fornatural rubber condoms. Higher the molecular weight between crosslinks,lower is the crosslink density.

The data presented indicates that the process of the present inventionresults in synthetic polyisoprene condoms that have very consistentmolecular weight between crosslinks. Since Durex polyisoprene condomshave a higher value of molecular weight between crosslinks, thecrosslink density is lower than that produced by the present process.The molecular weight between crosslinks for the condoms according to thepresent invention is comparable to that of natural rubber and hasadequate mechanical properties.

TABLE 7 Molecular weight Crosslink between crosslinks Density n Condom(g/mol) mol/cm³ Synthetic Polyisoprene Condom Set #1* 6535 0.000141Synthetic Polyisoprene Condom Set#2* 6537 0.000141 SyntheticPolyisoprene Condom Set#3* 6754 0.000136 Durex Synthetic Polyisoprenecondom 8955 0.000103 Natural Rubber regular condom 5788 0.0000159*Condoms manufactured according to the subject invention.

FIG. 1 shows a transmission electron micrograph of a pre-vulcanized andpost-vulcanized synthetic polyisoprene condom latex taken from themiddle portion of the condom thickness. The sample was prepared usingthe following procedure. Sample regions of the condom sample were takenand extracted in cold acetone overnight to remove any low molecularweight materials that may subsequently interfere with the styrenepolymerization process. The samples were then dried for approximately 48hrs at below 40° C. to remove any solvent traces. The extracted filmswere then swollen overnight in styrene solution containing 1 wt %benzoyl peroxide initiator and 2 wt % dibutylphthalate plasticizer toaid sectioning. The swollen films were then placed in capsules withexcess styrene solution and heated at 70° C. until the styrene had fullypolymerized. The styrene-swollen, polymerized samples were sectioned byultramicrotomy at room temperature. By leaving some polystyrene attachedto the surfaces of each condom, it was possible to prepare ultra-thinsections that contained the entire width of each condom. The sectionswere carefully relaxed by exposure to low levels of xylene vapor andtransferred to Transmission Electron Microscopy (TEM) grids. Thesections were then stained in osmium tetroxide vapor for one hour andexamined by TEM. Osmium tetroxide reacts with carbon-carbon double bondsand, therefore, it imparts a dark stain to polymers containingunsaturated groups, while leaving the polystyrene unstained. The figureshows at 10 the original synthetic polyisoprene particles showinguniform distribution of cross-link networks. The intersection of theseparticles is shown at 11, and it shows a similar distribution ofcross-link networks indicated by uniformity of dark stains, indicatingthat the synthetic polyisoprene latex film is cross-linked at thesynthetic polyisoprene particle level and at intersections. Thepolystyrene remnants are seen at 13. The overall particle size isapproximately 0.8 microns. This homogeneously cured, syntheticpolyisoprene results in improved tensile strength at break, superiorelongation, and tear properties.

In view of the above, the present invention provides an article madefrom the above-described surfactant-stabilized, pre-vulcanized,synthetic polyisoprene latex emulsion composition. The article is freefrom defects and has a stretch to failure of at least about 600%. Table5 shows an elongation of over 1000% at failure. The article hasintra-particle and inter-particle crosslinking and under transmissionelectron microscopy (TEM) a uniform distribution of dark stains with adeviation of less than about 5% from one location to other within theTEM micrograph. The synthetic polyisoprene article is preferably acondom or a glove.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a,” “an,” “the,” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. Recitation of ranges of values herein are merely intended toserve as a shorthand method of referring individually to each separatevalue falling within the range, unless otherwise indicated herein, andeach separate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context. The use of any and allexamples, or exemplary language (e.g., “such as”) provided herein, isintended merely to illuminate better the invention and does not pose alimitation on the scope of the invention, unless otherwise claimed. Nolanguage in the specification should be construed as indicating anynon-claimed element as essential to the practice of the invention.

What is claimed is:
 1. A synthetic, dip-formed polyisoprene elastomericcondom comprising: synthetic polyisoprene particles, said syntheticpolyisoprene particles bonded to each other through intra-polyisopreneparticle crosslinks and inter-polyisoprene particle crosslinks; whereinthe intra-polyisoprene particle crosslinks and the inter-polyisopreneparticle crosslinks are such that the molecular weight is less thanabout 8000 g/mol between the crosslinks.
 2. A condom of claim 1, whereinthe condom has an elongation at break of at least 945%.
 3. The condom ofclaim 2, wherein the condom exhibits a fracture surface with an absenceof scanning electron microscope-viewable intra-polyisoprene particle andinter-polyisoprene particle features when ruptured at room temperature.4. A condom of claim 2, wherein the condom has a tensile strength of atleast 30 MPa.
 5. A condom of claim 1, wherein the intra-polyisopreneparticle crosslinks and the inter-polyisoprene particle crosslinks aresuch that the molecular weight is less than about 7500 g/mol between thecrosslinks.
 6. The condom of claim 5, wherein the condom has anelongation at break of at least 945%.
 7. The condom of claim 6, whereinthe condom exhibits a fracture surface with an absence of scanningelectron microscope-viewable intra-polyisoprene particle andinter-polyisoprene particle features when ruptured at room temperature.8. The condom of claim 6, wherein the condom has a tensile strength ofat least 30 MPa.
 9. A condom of claim 1, wherein the intra-polyisopreneparticle crosslinks and the inter-polyisoprene particle crosslinks aresuch that the molecular weight is less than about 7000 g/mol between thecrosslinks.
 10. The condom of claim 9, wherein the condom has anelongation at break of at least 945%.
 11. The condom of claim 10,wherein the condom exhibits a fracture surface with an absence ofscanning electron microscope-viewable intra-polyisoprene particle andinter-polyisoprene particle features when ruptured at room temperature.12. The condom of claim 10, wherein the condom has a tensile strength ofat least 30 MPa.
 13. A condom of claim 1, wherein the intra-polyisopreneparticle crosslinks and the inter-polyisoprene particle crosslinks aresuch that the molecular weight is less than about 8900 g/mol between thecrosslinks.
 14. A condom of claim 1, wherein the intra-polyisopreneparticle crosslinks and the inter-polyisoprene particle crosslinks aresuch that the molecular weight is less than about 8800 g/mol between thecrosslinks.
 15. A condom of claim 1, wherein the intra-polyisopreneparticle crosslinks and the inter-polyisoprene particle crosslinks aresuch that the molecular weight is less than about 8700 g/mol between thecrosslinks.
 16. A condom of claim 1, wherein the intra-polyisopreneparticle crosslinks and the inter-polyisoprene particle crosslinks aresuch that the molecular weight is less than about 8600 g/mol between thecrosslinks.
 17. A condom of claim 1, wherein the intra-polyisopreneparticle crosslinks and the inter-polyisoprene particle crosslinks aresuch that the molecular weight is less than about 8400 g/mol between thecrosslinks.
 18. A condom of claim 1, wherein the intra-polyisopreneparticle crosslinks and the inter-polyisoprene particle crosslinks aresuch that the molecular weight is less than about 8300 g/mol between thecrosslinks.
 19. A condom of claim 1, wherein the intra-polyisopreneparticle crosslinks and the inter-polyisoprene particle crosslinks aresuch that the molecular weight is less than about 8200 g/mol between thecrosslinks.
 20. A condom of claim 1, wherein the intra-polyisopreneparticle crosslinks and the inter-polyisoprene particle crosslinks aresuch that the molecular weight is less than about 8100 g/mol between thecrosslinks.